1. Field of the Invention
The present invention relates to a transmission line apparatus for transmitting an analog RF signal or a digital signal that has a frequency falling within a microwave band or a milliwave band.
2. Description of the Related Art
FIG. 15A illustrates the cross-sectional structure of a microstrip line, which is used as conventional transmission line. In FIG. 15A, a signal strip 102 is formed on a substrate 101 made of a dielectric material or a semiconductor, while a ground conductor layer 105 is formed on the back surface of the substrate 101. When radio frequency power is supplied to this microstrip line, an electric field is generated between the signal strip 102 and the ground conductor layer 105. On the other hand, a magnetic field is generated perpendicularly to the electric lines of force so as to surround the signal strip 102. And the radio frequency power propagates in the longitudinal direction in which this electromagnetic field crosses the width direction of the signal strip 102 at right angles. In the microstrip line, the signal strip 102 and the ground conductor layer 105 do not have to be formed on the surface and back surface of the substrate 101. Alternatively, if the substrate 101 is implemented as a multilayer circuit board, then the signal strip 102 and ground conductor layer 105 may be formed on an inner conductor plane of the circuit board.
The transmission line described above is used to transmit a single-ended signal. However, if two microstrip wiring circuit structures are arranged parallel to each other as shown in the cross-sectional view of FIG. 15B and if signals of mutually inverse phases are supplied to those two lines, the pair of microstrip lines may be used as a differential transmission line. In that case, since signals of inverse phases are supplied to, and transmitted through, a pair of signal strips 102a and 102b, a virtual ground plane is produced between the signal strips 102a and 102b. Therefore, a differential transmission line could be formed without the ground conductor layer 105, theoretically speaking. Actually, however, a differential transmission line often has a circuit configuration including the ground conductor layer.
As shown in FIGS. 16A and 16B, two or more signal strips 102a and 102b are often densely arranged side by side and parallel to each other in an analog circuit or a high-speed digital circuit. In FIGS. 16A and 16B, W is the distance between two or more signal strips 102a and 102b, G is the gap between the signal strips and D shows the distance between the center of the signal strips. As a result, crosstalk will often occur between the adjacent transmission lines, thus causing the problem of a decreased degree of isolation.
Japanese Patent Application Laid-Open Publication No. 2001-257509 attributes the crosstalk phenomenon to a difference in intensity between a negative induced voltage caused by the mutual inductance of a pair of transmission lines and a positive induced voltage caused by the mutual capacitance thereof. An equivalent circuit of a pair of transmission lines per unit length is defined by the series inductance L, ground capacitance C and mutual inductance M of the transmission lines and the mutual capacitance Cm between the transmission lines as shown in FIG. 17.
If an RF voltage Vo is supplied to the input terminal 106a shown in FIG. 17 so as to travel through the signal strip 102a, then induced voltages Vi and Vc are caused in its adjacent signal strip 102b due to mutual inductance and mutual capacitance, respectively. Vi and Vo have the opposite signs but Vc and Vo have the same sign. Terminals 106b and 106c are shown in FIG. 17. As a result, a far-end crosstalk voltage with an intensity Vc-Vi is produced at a far-end crosstalk terminal 106d. On normal conditions on which signal strips are arranged densely, Vc has a lower intensity than Vi, and therefore, the far-end crosstalk voltage eventually has the sign opposite to that of the input voltage Vo. Such a crosstalk problem is non-negligible if the coupled line length Lcp of multiple adjacent transmission lines is equal to or greater than one-fourth of the effective wavelength at the transmission signal frequency.
Japanese Patent Application Laid-Open Publication Nos. 2001-257509 and 2004-015534 disclose methods for suppressing this crosstalk phenomenon. Both of those methods follow the principle of offsetting the difference in intensity between Vi and Vc by decreasing an additional induced voltage Vadd and increasing Vc. That is to say, the far-end crosstalk is minimized by newly providing a so-called “additional capacitance element” that increases the mutual capacitance between the transmission lines. As shown in the equivalent circuit diagram of FIG. 18, a capacitance Ca between the transmission lines is newly added to the equivalent circuit of the pair of transmission lines shown in FIG. 17.
As to the method of implementing Ca, Japanese Patent Application Laid-Open Publication No. 2001-257509 discloses two specific configurations as examples. According to a first one of the two configurations, Ca to be inserted between first and second signal strips 102a and 102b is implemented by capacitors 317 as shown in FIG. 19. On the other hand, according to the second configuration, the gap between the signal strips is narrowed by broadening the respective widths of the signal strips of the transmission lines.
Japanese Patent Application Laid-Open Publication No. 2004-015534 discloses configurations that introduce additional members called “crosstalk suppressing components”. Specifically, as shown in the perspective view of FIG. 20, crosstalk suppressing components 319, each partially making a plane contact with the first and second signal strips 102a and 102b and connecting the first and second signal strips 102a and 102b together with a conductor, are arranged. A sufficient capacitance is produced in the intersecting area between each of the crosstalk suppressing components 319 and the first or second signal strip 102a, 102b. And those capacitances are connected in series together, thus achieving the same effects as those achieved by Japanese Patent Application Laid-Open Publication No. 2001-257509. Examples of the crosstalk suppressing components 319 disclosed in Japanese Patent Application Laid-Open Publication No. 2004-015534 include a rectangular one as shown in the top view of FIG. 21A and a one with a shape that is designed to include a phase advancing area 313c between a capacitance area 313a intersecting with the first signal strip and a capacitance area 313b intersecting with the second signal strip as shown in the top view of FIG. 21B. In the following description, the crosstalk suppressing components shown in FIGS. 21A and 21B will be respectively referred to herein as Configuration Examples Nos. 1 and 2 of Japanese Patent Application Laid-Open Publication No. 2004-015534. Their equivalent circuits are also disclosed in Japanese Patent Application Laid-Open Publication No. 2004-015534. As shown in FIG. 22, the additional capacitance Ca of the equivalent circuit disclosed in Japanese Patent Application Laid-Open Publication No. 2001-257509 is implemented by a circuit in which capacitances C1 and C2 are connected in series together. In Configuration Example No. 2 of Japanese Patent Application Laid-Open Publication No. 2004-015534, inductance Lp is intentionally added between the capacitances C1 and C2.
Those conventional pairs of transmission lines that are specially designed to suppress the crosstalk, however, have the following three problems, for example, and are actually unable to suppress the crosstalk effectively.
Firstly, Configuration Example No. 1 of Japanese Patent Application Laid-Open Publication No. 2001-257509 needs additional external circuit components in capacitors, thus requiring increased component and assembling costs. The crosstalk suppressing components disclosed in Japanese Patent Application Laid-Open Publication No. 2004-015534 also have a similar problem. Also, even though the thickness of the circuit should be reduced as much as possible, the thickness of the chip component needs to be added to that of the circuit board. As a result, the volume of the circuit increases. Furthermore, if the external chip components or the external components called the “crosstalk suppressing components” are used in a pair of transmission lines to transmit a high-speed signal, then the transmission performance itself varies due to some variations in either assembling or in the characteristics of the chip components.
Thus, first of all, a transmission line apparatus that can suppress the crosstalk using some component that has been integrated with the circuit board, not the external components, needs to be provided. To realize high-speed transmission and connection with functional components, the transmission lines are preferably formed on the surface of a substrate. That is why a method of suppressing crosstalk with an additional capacitance element arranged on either the surface of the substrate along with the transmission lines or on an inner surface of the substrate needs to be provided.
Secondly, if an additional capacitance element were arranged inside a circuit as taught in Japanese Patent Application Laid-Open Publication No. 2001-257509 or 2004-015534, then the crosstalk characteristic should be improved in an ideal equivalent circuit. In an actual circuit, however, it is difficult to improve that characteristic. Thus, the crosstalk suppressing effects need to be achieved in an actual circuit by correcting the imperfections of the principles of Japanese Patent Application Laid-Open Publication Nos. 2001-257509 and 2004-015534.
Thirdly, according to a method of increasing the mutual capacitance between the signal lines by locally increasing the line widths of the signal lines as disclosed in Japanese Patent Application Laid-Open Publication No. 2001-257509, the characteristic impedances of the lines change, thus deteriorating the transmission characteristic. That is why a structure that never deteriorates the transmission characteristic needs to be provided.
In order to overcome the problems described above, an object of the present invention is to provide a transmission line apparatus that can improve the crosstalk characteristic of an actual circuit without using any additional component and with the influence of parasitic components of the circuit elements taken into consideration and that never deteriorates the transmission characteristic.